drm/i915/bdw: Ack interrupts before handling them (GEN8)

Otherwise, we might receive a new interrupt before we have time to
ack the first one, eventually missing it.

The right order should be:

1 - Disable Master Interrupt Control.
2 - Find the category of interrupt that is pending.
3 - Find the source(s) of the interrupt and clear the Interrupt Identity bits (IIR)
4 - Process the interrupt(s) that had bits set in the IIRs.
5 - Re-enable Master Interrupt Control.

Without an atomic XCHG operation with mmio space, the above merely reduces the window
in which we can miss an interrupt (especially when you consider how heavyweight the
I915_READ/I915_WRITE operations are).

Spotted by Bob Beckett <robert.beckett@intel.com>.

v2: Add warning to commit message and comments to the code as per Chris Wilson's request.

v3: Improve the source code comment.
Signed-off-by: NOscar Mateo <oscar.mateo@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -1465,6 +1465,7 @@ static irqreturn_t gen8_gt_irq_handler(struct drm_device *dev,
 	if (master_ctl & (GEN8_GT_RCS_IRQ | GEN8_GT_BCS_IRQ)) {
 		tmp = I915_READ(GEN8_GT_IIR(0));
 		if (tmp) {
+			I915_WRITE(GEN8_GT_IIR(0), tmp);
 			ret = IRQ_HANDLED;
 			rcs = tmp >> GEN8_RCS_IRQ_SHIFT;
 			bcs = tmp >> GEN8_BCS_IRQ_SHIFT;
@@ -1472,7 +1473,6 @@ static irqreturn_t gen8_gt_irq_handler(struct drm_device *dev,
 				notify_ring(dev, &dev_priv->ring[RCS]);
 			if (bcs & GT_RENDER_USER_INTERRUPT)
 				notify_ring(dev, &dev_priv->ring[BCS]);
-			I915_WRITE(GEN8_GT_IIR(0), tmp);
 		} else
 			DRM_ERROR("The master control interrupt lied (GT0)!\n");
 	}
@@ -1480,6 +1480,7 @@ static irqreturn_t gen8_gt_irq_handler(struct drm_device *dev,
 	if (master_ctl & (GEN8_GT_VCS1_IRQ | GEN8_GT_VCS2_IRQ)) {
 		tmp = I915_READ(GEN8_GT_IIR(1));
 		if (tmp) {
+			I915_WRITE(GEN8_GT_IIR(1), tmp);
 			ret = IRQ_HANDLED;
 			vcs = tmp >> GEN8_VCS1_IRQ_SHIFT;
 			if (vcs & GT_RENDER_USER_INTERRUPT)
@@ -1487,7 +1488,6 @@ static irqreturn_t gen8_gt_irq_handler(struct drm_device *dev,
 			vcs = tmp >> GEN8_VCS2_IRQ_SHIFT;
 			if (vcs & GT_RENDER_USER_INTERRUPT)
 				notify_ring(dev, &dev_priv->ring[VCS2]);
-			I915_WRITE(GEN8_GT_IIR(1), tmp);
 		} else
 			DRM_ERROR("The master control interrupt lied (GT1)!\n");
 	}
@@ -1495,10 +1495,10 @@ static irqreturn_t gen8_gt_irq_handler(struct drm_device *dev,
 	if (master_ctl & GEN8_GT_PM_IRQ) {
 		tmp = I915_READ(GEN8_GT_IIR(2));
 		if (tmp & dev_priv->pm_rps_events) {
-			ret = IRQ_HANDLED;
-			gen8_rps_irq_handler(dev_priv, tmp);
 			I915_WRITE(GEN8_GT_IIR(2),
 				   tmp & dev_priv->pm_rps_events);
+			ret = IRQ_HANDLED;
+			gen8_rps_irq_handler(dev_priv, tmp);
 		} else
 			DRM_ERROR("The master control interrupt lied (PM)!\n");
 	}
@@ -1506,11 +1506,11 @@ static irqreturn_t gen8_gt_irq_handler(struct drm_device *dev,
 	if (master_ctl & GEN8_GT_VECS_IRQ) {
 		tmp = I915_READ(GEN8_GT_IIR(3));
 		if (tmp) {
+			I915_WRITE(GEN8_GT_IIR(3), tmp);
 			ret = IRQ_HANDLED;
 			vcs = tmp >> GEN8_VECS_IRQ_SHIFT;
 			if (vcs & GT_RENDER_USER_INTERRUPT)
 				notify_ring(dev, &dev_priv->ring[VECS]);
-			I915_WRITE(GEN8_GT_IIR(3), tmp);
 		} else
 			DRM_ERROR("The master control interrupt lied (GT3)!\n");
 	}
@@ -2241,36 +2241,36 @@ static irqreturn_t gen8_irq_handler(int irq, void *arg)
 	I915_WRITE(GEN8_MASTER_IRQ, 0);
 	POSTING_READ(GEN8_MASTER_IRQ);
 
+	/* Find, clear, then process each source of interrupt */
+
 	ret = gen8_gt_irq_handler(dev, dev_priv, master_ctl);
 
 	if (master_ctl & GEN8_DE_MISC_IRQ) {
 		tmp = I915_READ(GEN8_DE_MISC_IIR);
+		if (tmp) {
+			I915_WRITE(GEN8_DE_MISC_IIR, tmp);
+			ret = IRQ_HANDLED;
 			if (tmp & GEN8_DE_MISC_GSE)
 				intel_opregion_asle_intr(dev);
-		else if (tmp)
+			else
 				DRM_ERROR("Unexpected DE Misc interrupt\n");
+		}
 		else
 			DRM_ERROR("The master control interrupt lied (DE MISC)!\n");
-
-		if (tmp) {
-			I915_WRITE(GEN8_DE_MISC_IIR, tmp);
-			ret = IRQ_HANDLED;
-		}
 	}
 
 	if (master_ctl & GEN8_DE_PORT_IRQ) {
 		tmp = I915_READ(GEN8_DE_PORT_IIR);
+		if (tmp) {
+			I915_WRITE(GEN8_DE_PORT_IIR, tmp);
+			ret = IRQ_HANDLED;
 			if (tmp & GEN8_AUX_CHANNEL_A)
 				dp_aux_irq_handler(dev);
-		else if (tmp)
+			else
 				DRM_ERROR("Unexpected DE Port interrupt\n");
+		}
 		else
 			DRM_ERROR("The master control interrupt lied (DE PORT)!\n");
-
-		if (tmp) {
-			I915_WRITE(GEN8_DE_PORT_IIR, tmp);
-			ret = IRQ_HANDLED;
-		}
 	}
 
 	for_each_pipe(pipe) {
@@ -2280,6 +2280,9 @@ static irqreturn_t gen8_irq_handler(int irq, void *arg)
 			continue;
 
 		pipe_iir = I915_READ(GEN8_DE_PIPE_IIR(pipe));
+		if (pipe_iir) {
+			ret = IRQ_HANDLED;
+			I915_WRITE(GEN8_DE_PIPE_IIR(pipe), pipe_iir);
 			if (pipe_iir & GEN8_PIPE_VBLANK)
 				intel_pipe_handle_vblank(dev, pipe);
 
@@ -2303,10 +2306,6 @@ static irqreturn_t gen8_irq_handler(int irq, void *arg)
 					  pipe_name(pipe),
 					  pipe_iir & GEN8_DE_PIPE_IRQ_FAULT_ERRORS);
 			}
-
-		if (pipe_iir) {
-			ret = IRQ_HANDLED;
-			I915_WRITE(GEN8_DE_PIPE_IIR(pipe), pipe_iir);
 		} else
 			DRM_ERROR("The master control interrupt lied (DE PIPE)!\n");
 	}
@@ -2318,13 +2317,13 @@ static irqreturn_t gen8_irq_handler(int irq, void *arg)
 		 * on older pch-split platforms. But this needs testing.
 		 */
 		u32 pch_iir = I915_READ(SDEIIR);
-
-		cpt_irq_handler(dev, pch_iir);
-
 		if (pch_iir) {
 			I915_WRITE(SDEIIR, pch_iir);
 			ret = IRQ_HANDLED;
-		}
+			cpt_irq_handler(dev, pch_iir);
+		} else
+			DRM_ERROR("The master control interrupt lied (SDE)!\n");
+
 	}
 
 	I915_WRITE(GEN8_MASTER_IRQ, GEN8_MASTER_IRQ_CONTROL);